It is known that paper pulp made by recycling used paper must undergo numerous filtering, straining and separating operations. These are generally in strainers and classifiers having cylindrical sieves made of sheetmetal of stainless steel and whose perforations are holes or slots.
The quality of the filtration depends to a large extent on the fineness of the slots (of some tenths of a millimeter) and on the control of this calibration.
According to a classic technique the perforations made by milling in a thick sheet but the cost is high.
In order to lower this cost sieves are made by juxtaposition of U-shaped elements. The perforations are made in a sheet of thin thickness, and are then easier and faster to make.
The U-shaped elements are either flat forming a closed rim and are stacked on one another, or wound spirally. Thus a sieve is obtained by winding on itself a single element of U-shaped profile, as is described in EP 89.402.248.2.
However, with sieves thus made difficulties are encountered in assembling the sieve on the end rims which will center it and maintain it in the apparatus.
In fact, when the sieve is made by winding of a U-shaped section, the ends of the sieve do not present a plane perpendicular to the axis of the sieve on which the rim can easily abut and be welded or screwed.
And when the sieve is made by spiral winding of flat stock on edge, on a thin perforated sheetmetal, this helicoidal flat stock does not facilitate the mounting of an end rim which must remain perpendicular to the axis of the cylinder.